It has long been known to use liquid crystals having a smectic phase in display devices which may be addressed by using infrared radiation (U.S. Pat. No. 3,796,999). The combination of infrared addressing with electrical stimulation to produce data storage devices having memory erase is also known as shown in U.S. Pat. No. 3,836,243. Several variations of this type of device have been disclosed which utilize a thermally induced smectic to nematic, or smectic to cholesteric phase shift (Kahn, et al.; Performance and Characteristics of Smectic Liquid Crystal Storage Displays in Liquid Crystal Devices, T. Kallard, Ed., Optosonic Press, 1973).
Recently, several new types of liquid crystal displays have been disclosed which utilize a pleochroic dye in guest-host arrangement with a liquid crystal host having a smectic to nematic mesophase. Such displays are disclosed in U.S. Pat. No. 4,196,974.
Still more recently, a particularly advantageous display known as a smectic-cholesteric display has been described in U.S. Ser. No. 251,247, filed Apr. 6, 1981. Briefly, this display comprises a typical transmissive or reflective liquid crystal display having appropriate front and back electrode patterns and featuring a smectic liquid crystal medium including at least one pleochroic dye of high order parameter and at least one cholesteric liquid crystal compound. The liquid crystal medium is thermally sensitive and has a transition between an upper thermal cholesteric state and a lower smectic phase. The device is operated by heating the medium to its upper thermal cholesteric state. The medium is then rapidly cooled while a portion of the mixture is addressed by the application of an electric field. A homeotropic light transmissive state is developed in the portions of the mixture which are addressed by the electric field, while a light absorbing state develops in the unaddressed portions of the mixture. The pleochroic dye in the portion of the mixture which is unaddressed, absorbs incident light passing through the medium; the liquid crystal medium acting as a vehicle to orient the pleochroic dye molecules into a light absorbing position. Electrodes are provided adjacent the liquid crystal medium, and heating electrodes are also provided to heat the medium to the upper cholesteric phase. By selective activation of the electrodes, information can be readily displayed.
Due to its operating mechanism, the smectic-cholesteric display does not require the use of auxiliary polarizers. However, the display has a number of inherent disadvantages. Significant amounts, e.g. 5-15% by weight, of the cholesteric liquid crystal and dichroic dye additives must be incorporated into the smectic liquid crystal medium. These "foreign" additives are typically chemically and/or photochemically unstable in liquid crystal display systems and their addition introduces a potential source of harmful decomposition products into the display. In addition, it is well known that liquid crystal systems comprising chemically dissimilar mixtures of liquid crystals have properties which vary nonlinearly and unpredictably with composition and temperature.